In the manufacture of molded carbonaceous materials such as carbon electrodes, calcined coke is generally employed as the starting material. As the coke possesses no natural adhesiveness, it must be bound together with a compatible material. In the manufacture of carbon electrodes, the coke is usually ground, mixed with a binder, molded and then baked to carbonize the binder. Binders employed for these purposes must be sufficiently fluid at the temperatures at which they are mixed with the solid coke in order to completely wet and penetrate the coke. Furthermore, the binder should be relatively stable when subjected to temperatures as high as about 200.degree. C. Because of these stringent requirements, commercially employed pitch binders have been manufactured almost exclusively from selected coal-tar products.
Binders employed in the production of molded carbonaceous articles should generally comprise a stable hydrocarbon mixture of uniform consistency and quality, and should be substantially free from contaminants. Among the contaminants which have prevented the use of petroleum-derived pitch in carbonaceous articles, are coke-like materials. The term "coke" or "coke-like materials" as employed herein denotes a hydrocarbon having a carbon to hydrogen atomic ratio of about 2.0 or higher, and is generally produced as a result of excessive polymerization and condensation reactions, which are necessary for obtaining a pitch binder having the desired properties. A satisfactory pitch binder, suitable for use in the manufacture of carbon electrodes, can hold in stable suspension only about 2.5 weight percent of coke-like materials. At coke concentrations greater than about 2.5 weight percent, it is found that the excess coke-like material tends to agglomerate and to separate from the pitch, producing a non-uniform product and providing a non-uniform binder. This coke-like material has been found to have poor binding properties and does not contribute to the desired characteristics of the finished product. The presence of excess coke in petroleum-derived pitch, therefore, is one of the main factors causing migration during baking treatment of carbon electrodes, and resulting in areas of unbonded carbon in the finished article. In this condition, a carbon electrode employing a petroleum pitch binder, is produced having uneven mechanical strength, and is variable in conductivity. For these reasons, petroleum pitches have not been considered as satisfactory binders, since they fail to meet the stringent requirements of commercially desirable pitch binders. Thus, commercial pitch binders generally have been produced almost exclusively from coal tar. Moreover, in some applications the presence of excessive sulfur in the aforementioned electrodes causes deleterious effects during ore reduction.
One method contemplated by the prior art for preparing a pitch binder from petroleum-derived hydrocarbons involves thermally cracking a high-boiling hydrocarbon fraction, and passing the hot cycle residue into a soaking tank of substantial capacity. The flow of the cycle residue is regulated through the soaking tank, so that the residence time for any given increment of feed-stock ranges from about 3 to about 5 hours. The initial pitch thus produced is not considered satisfactory, and is recycled through the soaking tank to improve its quality. However, over an extended soaking time period, an excessive amount of coke-like material is produced in the upgrading of the pitch product, and some of the excess coke separates and tends to accumulate at the bottom of the soaking tank. This condition requires an extensive cleaning operation to remove undesired accumulated coke. In addition, the excess coke thus produced contaminates the pitch product and causes the undesirable characteristics of a non-uniform pitch binder. Furthermore, the extended soaking operation of the prior art, is found to be a time consuming and expensive method for improving the quality of the pitch binder.
U.S. Pat. No. 3,707,388 describes the preparation of an improved asphaltic binder composition by a controlled air-blowing polymerization of a petroleum asphalt having a specific gravity of at least about 1.1 and a benzene-insoluble content of not more than 12 weight percent.
U.S. Pat. No. 3,725,240 describes a method for producing an improved asphaltic binder composition for carbon electrodes, by a multistep procedure which involves (1) conversion of a petroleum refinery cycle stock by thermal treatment to vis-broken cycle stock, (2) fractionation and separation of a heavy fraction from the vis-broken cycle stock, and (3) air-blowing of the heavy fraction under conditions which yield a pitch suitable for use as a binder for electrode coke. U.S. Pat. No. 2,340,847 describes a tar binder which is a pitch residue of coal tar (from which toluene-insoluble fractions have been removed) in combination with a high boiling oil (e.g., creosote).
U.S. Pat. No. 3,147,205 teaches an improved method for upgrading coal tar for application as an electrode binder. The method consists of the rejection of inorganic components and quinoline-insoluble organic components from coal tar by treatment of the coal tar with benzene under conditions of high temperature and pressure.
There remains a need for new and improved petroleum-derived asphalt compositions which have superior properties for use as binders for molded carbonaceous materials. Further, the development of such petroleum-derived asphalt compositions must contend with recent international economic developments which have resulted in a several fold increase in the price of raw petroleum.
Accordingly, it is a main object of the present invention to provide a petroleum-derived asphalt composition which has superior properties for use as a binder in carbon electrodes.
It is another object of the present invention to provide a process for upgrading low value petroleum refinery residual streams into high value asphalt compositions, without a required air-blowing polymerization treatment.
It is another object of the present invention to provide a petroleum asphaltic composition which contains a major proportion of highly aromatic organic components derived from coal and/or wood.
It is a further object of the present invention to provide an asphaltic composition with a pitch-like consistency which has a low sulfur content and exhibits high binding strength suitable for use in the manufacture of carbon electrodes.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.